Add --copy-links, use stat instead of lstat

To properly sync symlinks, use stat instead of lstat, otherwise we get the size
of the link (instead of the file) and we end up constantly re-pushing
everything. Also add a --copy-links/-L option that enables syncing of symlinks,
similarly to rsync.

adb-sync

@@ -36,6 +36,9 @@ class OSLike(object):
   def lstat(self, path: bytes) -> os.stat_result:  # os's name, so pylint: disable=g-bad-name
     raise NotImplementedError('Abstract')
 
+  def stat(self, path: bytes) -> os.stat_result:  # os's name, so pylint: disable=g-bad-name
+    raise NotImplementedError('Abstract')
+
   def unlink(self, path: bytes) -> None:  # os's name, so pylint: disable=g-bad-name
     raise NotImplementedError('Abstract')
 
@@ -259,6 +262,23 @@ class AdbFileSystem(GlobLike, OSLike):
         return statdata
     raise OSError('No such file or directory')
 
+  def stat(self, path: bytes) -> os.stat_result:  # os's name, so pylint: disable=g-bad-name
+    """Stat a file."""
+    if path in self.stat_cache and not stat.S_ISLNK(
+        self.stat_cache[path].st_mode):
+      return self.stat_cache[path]
+    with Stdout(
+        self.adb +
+        [b'shell', b'ls -aldL %s' % (self.QuoteArgument(path),)]) as stdout:
+      for line in stdout:
+        if line.startswith(b'total '):
+          continue
+        line = line.rstrip(b'\r\n')
+        statdata, _ = self.LsToStat(line)
+        self.stat_cache[path] = statdata
+        return statdata
+    raise OSError('No such file or directory')
+
   def unlink(self, path: bytes) -> None:  # os's name, so pylint: disable=g-bad-name
     """Delete a file."""
     if subprocess.call(
@@ -316,13 +336,15 @@ class AdbFileSystem(GlobLike, OSLike):
       raise OSError('pull failed')
 
 
-def BuildFileList(fs: OSLike, path: bytes,
+def BuildFileList(fs: OSLike, path: bytes, follow_links: bool,
                   prefix: bytes) -> Iterable[Tuple[bytes, os.stat_result]]:
   """Builds a file list.
 
   Args:
     fs: File system provider (can be os or AdbFileSystem()).
     path: Initial path.
+    follow_links: Whether to follow symlinks while iterating. May recurse
+      endlessly.
     prefix: Path prefix for output file names.
 
   Yields:
@@ -330,6 +352,9 @@ def BuildFileList(fs: OSLike, path: bytes,
     Directories are yielded before their contents.
   """
   try:
+    if follow_links:
+      statresult = fs.stat(path)
+    else:
       statresult = fs.lstat(path)
   except OSError:
     return
@@ -342,9 +367,12 @@ def BuildFileList(fs: OSLike, path: bytes,
     for n in files:
       if n == b'.' or n == b'..':
         continue
-      for t in BuildFileList(fs, path + b'/' + n, prefix + b'/' + n):
+      for t in BuildFileList(fs, path + b'/' + n, follow_links,
+                             prefix + b'/' + n):
         yield t
-  elif stat.S_ISREG(statresult.st_mode) or stat.S_ISLNK(statresult.st_mode):
+  elif stat.S_ISREG(statresult.st_mode):
+    yield prefix, statresult
+  elif stat.S_ISLNK(statresult.st_mode) and not follow_links:
     yield prefix, statresult
   else:
     logging.info('Unsupported file: %r.', path)
@@ -444,7 +472,7 @@ class FileSyncer(object):
   def __init__(self, adb: AdbFileSystem, local_path: bytes, remote_path: bytes,
                local_to_remote: bool, remote_to_local: bool,
                preserve_times: bool, delete_missing: bool,
-               allow_overwrite: bool, allow_replace: bool,
+               allow_overwrite: bool, allow_replace: bool, copy_links: bool,
                dry_run: bool) -> None:
     self.local = local_path
     self.remote = remote_path
@@ -455,6 +483,7 @@ class FileSyncer(object):
     self.delete_missing = delete_missing
     self.allow_overwrite = allow_overwrite
     self.allow_replace = allow_replace
+    self.copy_links = copy_links
     self.dry_run = dry_run
     self.num_bytes = 0
     self.start_time = time.time()
@@ -480,8 +509,9 @@ class FileSyncer(object):
   def ScanAndDiff(self) -> None:
     """Scans the local and remote locations and identifies differences."""
     logging.info('Scanning and diffing...')
-    locallist = BuildFileList(cast(OSLike, os), self.local, b'')
-    remotelist = BuildFileList(self.adb, self.remote, b'')
+    locallist = BuildFileList(
+        cast(OSLike, os), self.local, self.copy_links, b'')
+    remotelist = BuildFileList(self.adb, self.remote, self.copy_links, b'')
     self.local_only, self.both, self.remote_only = DiffLists(
         locallist, remotelist)
     if not self.local_only and not self.both and not self.remote_only:
@@ -757,6 +787,11 @@ def main() -> None:
       action='store_true',
       help='Do not ever overwrite any '
       'existing files. Mutually exclusive with -f.')
+  parser.add_argument(
+      '-L',
+      '--copy-links',
+      action='store_true',
+      help='transform symlink into referent file/dir')
   parser.add_argument(
       '--dry-run',
       action='store_true',
@@ -797,6 +832,7 @@ def main() -> None:
   delete_missing = args.delete
   allow_replace = args.force
   allow_overwrite = not args.no_clobber
+  copy_links = args.copy_links
   dry_run = args.dry_run
   local_to_remote = True
   remote_to_local = False
@@ -830,7 +866,7 @@ def main() -> None:
     logging.info('Sync: local %r, remote %r', localpaths[i], remotepaths[i])
     syncer = FileSyncer(adb, localpaths[i], remotepaths[i], local_to_remote,
                         remote_to_local, preserve_times, delete_missing,
-                        allow_overwrite, allow_replace, dry_run)
+                        allow_overwrite, allow_replace, copy_links, dry_run)
     if not syncer.IsWorking():
       logging.error('Device not connected or not working.')
       return